Type B Elevator Safeties

In our last post we discussed Type A safeties, which are referred to as instantaneous applied safeties for elevators. In this article, we move onto Type B Safeties. First, I want to expand upon the use of safeties in general. Commonly, we think of safeties used on traction elevators, both high and low speed variations. Hydraulic elevators also incorporate safeties depending on the elevator's design. Some hydro's use traction ropes coupled with hydraulic rams; these units require safeties. Roped-hydros have become more and more common for the low-rise market in the past several years, but they have been in use for a long time. Please keep in mind that there are may variations of safeties and we are not going to cover them all. I can provide information upon request for specific applications.

Type B safeties are basically of two designs: flexible guide and wedge clamp. Both are activated via an over-speed governor and attached rope to the safeties assembly. We will discuss the over-speed governor in future articles.

Type B safeties are applied at a slower rate than Type A. These are used in higher speed elevators as the mass is moving at greater rate; to use instantaneous safeties would result in passenger and equipment damage. Type B safeties are similar to Type A safeties in that they are positioned on each side of the two guide rails and travel with the elevator. Both the wedge and flexible guide types of safeties make for a smooth application. The safety jaws are not the quick-grabbing knurled rollers as with Type A safeties, but are commonly two flat with grooved lengths of machined steel that "squeeze" together with the guide rail sandwiched between them. With the retardation of setting force, this makes for a smoother stop. I have attempted another rudimentary drawing of one common design of these safeties. It's how the squeeze is applied that makes the difference between the two. My two drawings hopefully help you to see what I am saying.

Code dictates the stopping distance tolerances by speeds. For instance, a 500-foot-per-minute (2.53 m/s) elevator traveling in the down direction is required no less than a minimum of 20 inches (508 mm) and a maximum distance of 68 inches (1727 mm) to come to a complete stop. Type B safeties are listed in current ASME code from 0 – 2,400 fpm. We commonly find a 500 fpm elevator in low to mid-rise passenger and freight elevators.

During testing of Type B safeties, the elevator is filled to capacity with test weights and in the down direction at contract speed or in some cases, depending on local codes and procedures, at the over-speed rate. The safeties are applied via the governor by either manually tripping the setting device or, in over-speed condition, the governor will do it all. We will do more on safety tests in the next couple of articles.

In the next, we will briefly look at Type C safeties and applications of safeties on counterweights.

- Joe